Electric Motors

3.2 An electric motor drive system is made up of five main components. Figure 3.1 shows a block diagram of an electric motor drive. The input to the drive is the power source. The power source is the energy for the system. Next is the power electronic converter. The electronic converter manipulates the voltage, current, […]

3.1 The power diode is the simplest, uncontrollable power electronic switch. A power diode is forward biased (on) when its current is positive and reverse biased (off) when its voltage is negative. A thyristor is a controllable three-terminal device. If a current pulse is applied to its gate, the thyristor can be turned on and […]

3.3 Shown in Fig. 3.2 is a separately excited DC motor controlled by a single-phase, half-wave controlled rectifier. This rectifier provides speed control for the separately excited DC motor by varying armature voltage and current. The steady state voltage and torque equations for a separately excited DC motor are FIGURE 3.1 Block diagram of an […]

3.4 The next electric drive discussed is a single-phase, full-wave controlled rectifier. This adjustable speed drive is similar to the single-phase, half-wave controlled rectifier. As an example, this rectifier is presented to control a separately excited DC motor. The single-phase, full-wave controlled rectifier consists of four thyristors. The increase in thyristors provides for better control […]

3.5 As mentioned earlier, three-phase induction motors make up the majority of the motors in the industry. This is because of their low FIGURE 3.9 Waveforms of a single-phase, full-wave controlled rectifier in CCM. cost, low maintenance, and high efficiency. To control any motor, a strong understanding of the motor equations is needed. Before discussing […]

3.6 DC/DC CONVERTERS Using DC/DC converters to control DC motors is very effective. The speed of the motor is controlled by the on and off time of the DC voltage. This is done through different switching schemes. The switching schemes are varied to produce the control needed. To understand the switching schemes, one must first […]

4.1 Traditionally, power factor has been defined as the ratio of the kilowatts of power divided by the kilovolt-amperes drawn by a load or system, or the cosine of the electrical angle between the kilowatts and kilovolt-amperes. However, this definition of power factor is valid only if the voltages and currents are sinusoidal. When the […]

The advantages of improving the equipment and system power factor are not as obvious as the advantages of improving the kilowatt power consumption. Improvement in the plant power factor can result in savings in kilowatt-hour power consumption due to lower distribution and transformer losses and, in many cases, a substantial reduction in the energy demand […]

4.2 The line current drawn by induction motors, transformers, and other inductive devices consists of two components: the magnetizing current and the power-producing current. The magnetizing current is that current required to produce the magnetic flux in the machine. This component of current creates a reactive power requirement that is measured in kilovolt-amperes reactive (kilovars, […]

4.3 A low power factor causes poor system efficiency. The total apparent power must be supplied by the electric utility. With a low power factor, or a high-kilovar component, additional generating losses occur throughout the system. Figures 4.3 and 4.4 illustrate the effect of the power factor on generator and transformer capacity. To discourage low-power […]